Late this afternoon, in a district court in Baltimore, Maryland, Under Armour filed a complaint against Nike, alleging trademark infringement. The trademark in question is Under Armour’s advertising phrase, “I Will,” which is part of a brand new marketing campaign. It is the first time that Under Armour has ever sued Nike.

launched an advertising campaign for its performance apparel, footwear, accessories, sporting goods, and related services that appropriates ‘I WILL’ prominently, repeatedly, and in a format and context that imitates Under Armour’s longstanding use of its iconic I WILL trademark/tagline for these same types of products and services.

Under Armour says that Nike has appropriated the phrase and has used it (and a variation of another UA marketing phrase, “Protect this house”) in various online and social media outlets. The Nike tag line: “I will protect my home court.” (Here's an example.)

Under Armour had revenues last year of $1.8 billion. Nike’s were $25 billion.

Under Armour founder and CEO, Kevin Plank, has long fashioned himself and his company as the scrappy underdog to Nike. “We prefer to battle our competitors in the marketplace and on the field of play,” Under Armour said in a statement.

A call to Nike has thus far received no response. A Nike spokesperson says the company has no comment at this time.